Year 2015: Day 04

README.md

@@ -7,13 +7,13 @@ Consistency is hard without proper goals to set a good (let's dare say "atomic")
 I'm also adding notes that may be useful if you're learning Ruby.
 
 Notes for solving:
-* [2015, up to day 01](year_2015.md)
+* [2015, up to day 04](year_2015.md)
 * [2023, up to day 03](year_2023.md)
 
 # How to use
 Install [RSpec](https://github.com/rspec/rspec-metagem) with `gem install rspec` and run `rspec` in the root directory.
 
 # Testing guidelines
-The tests are written with [Test-driven Development](https://en.wikipedia.org/wiki/Test-driven_development) principles by using the input and results provided by each exercise's examples.
+The tests are written with [Test-driven Development](https://en.wikipedia.org/wiki/Test-driven_development) principles by using the input and results provided by each exercise's samples. The  full input is then used in the tests, and the expected results are validated on the website.
 
 A [GitHub Action](https://docs.github.com/en/actions) is available to test the code as soon as it is pushed.

spec/year_2015/day_04_sample_one

@@ -0,0 +1,6 @@
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11

spec/year_2015/day_04_sample_two

@@ -0,0 +1,6 @@
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11

spec/year_2015/day_04_spec.rb

@@ -0,0 +1,62 @@
+require 'year_2015/day_04'
+
+describe Year2015::Day04 do
+  context 'Part 1' do
+    subject do
+      Year2015::Day04.new(File.read('spec/year_2015/day_04_sample_one'), true)
+    end
+
+    it 'finds winning numbers' do
+      [
+        [17, 48, 83, 86],
+        [32, 61],
+        [1, 21],
+        [84],
+        [],
+        []
+      ].each_with_index do |result, i|
+        expect(subject.lines[i].winning_numbers).to eq(result)
+      end
+    end
+
+    it 'calculates points' do
+      [8, 2, 2, 1, 0, 0].each_with_index do |result, i|
+        expect(subject.lines[i].to_i).to eq(result)
+      end
+    end
+
+    it 'gives a final result' do
+      expect(subject.to_i).to eq(13)
+    end
+  end
+
+  context 'Part 2' do
+    subject do
+      Year2015::Day04.new(File.read('spec/year_2015/day_04_sample_one'))
+    end
+
+    it 'distributes cards' do
+      [1, 2, 4, 8, 14, 1].each_with_index do |result, i|
+        expect(subject.lines[i].quantity).to eq(result)
+      end
+    end
+
+    it 'gives a final result' do
+      expect(subject.to_i).to eq(30)
+    end
+  end
+
+  context 'Results' do
+    subject do
+      File.read('spec/year_2015/day_04_input')
+    end
+
+    it 'correctly answers part 1' do
+      expect(Year2015::Day04.new(subject, true).to_i).to eq(17803)
+    end
+
+    it 'correctly answers part 2' do
+      expect(Year2015::Day04.new(subject).to_i).to eq(5554894)
+    end
+  end
+end

year_2015.md

@@ -46,4 +46,24 @@ Year2015::Day03
 
 The only thing to be wary of is on line 16: without the call to `#dup`, all of Santa's and Robo-Santa's positions will be overwritten, since Ruby's object model has a tendancy to pass references when you expect to pass values.
 
-Passing by value or reference is a really wonky subject, but this [blog post](https://robertheaton.com/2014/07/22/is-ruby-pass-by-reference-or-pass-by-value/) got nice examples that will get you started.
+Passing by value or reference is a really wonky subject, but this [blog post](https://robertheaton.com/2014/07/22/is-ruby-pass-by-reference-or-pass-by-value/) got nice examples that will get you started.
+
+## Day 04
+
+```
+Year2015::Day04
+  Part 1
+    finds winning numbers
+    calculates points
+    gives a final result
+  Part 2
+    distributes cards
+    gives a final result
+  Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+Part one is pretty easy. One useful method is the [`#&`](https://ruby-doc.org/core-3.0.1/Array.html#method-i-26) operator that takes two arrays and returns a new one with only the matching contents. The syntax is clearly inspired by [bit masking](https://en.wikipedia.org/wiki/Mask_(computing)), which is a subject you should look into if you've never heard of it.
+
+Part two is a bit more annoying and requires to do a two-pass, but nothing really improbable.

year_2015/day_04.rb

@@ -0,0 +1,51 @@
+class Year2015
+  class Day04
+    attr_reader :version, :lines
+
+    class Line
+      attr_accessor :quantity
+
+      def initialize(input_line)
+        @line = input_line
+        @winners = @line.split(':').last.split('|').first.scan(/\d+/).map(&:to_i)
+        @inputs  = @line.split('|').last.scan(/\d+/).map(&:to_i)
+        @quantity = 1
+      end
+
+      def winning_numbers
+        (@winners & @inputs).sort
+      end
+
+      def to_i
+        return 0 if winning_numbers.empty?
+
+        2**(winning_numbers.length - 1)
+      end
+    end
+
+    def initialize(input_file, input_part_one = false)
+      @version = input_part_one ? 1 : 2
+      @lines = input_file.chomp.split("\n").map do |input_line|
+        Line.new(input_line)
+      end
+      distribute_cards if version == 2
+    end
+
+    def distribute_cards
+      @lines.each_with_index do |line, i|
+        cards_won = line.winning_numbers.length
+        1.upto(cards_won) do |j|
+          next unless @lines[i + j]
+
+          @lines[i + j].quantity += line.quantity
+        end
+      end
+    end
+
+    def to_i
+      return @lines.map(&:quantity).inject(&:+) if version == 2
+
+      @lines.map(&:to_i).inject(&:+)
+    end
+  end
+end